Preparation of powder composition for making dispersion strengthened binary and higher nickel base alloys

ABSTRACT

METAL-REFRACTORY OXIDE POWDER COMPOSITIONS ADAPTED FOR POWDER METALLURGICAL FABRICATION OF WROUGHT DISPERSION STRENGTHENED NICKEL BASE BINARY AND HIGHER ALLOYS ARE PREPARED BY WET ATTRITOR GRINDING BLENDS OF VERY FINELY DIVIDED METAL POWDERS. THE BLENDS ARE FORMED OF COMPOSITE NICKEL-REFRACTORY OXIDE POWDER AND CHROMIUM, TUNGSTEN, MOLYBDENUM AND/OR COBALT POWDER HAVING A PARTICLE SIZE BELOW 1 MICRON. THE WET ATTRITOR GRINDING IS CONDUCTED FOR A TIME SUFFICIENT TO RPODUCE A HOMOGENEOUS POWDER MIXTURE AND TO EFFECT UNIFORM SPATIAL ISTRIBUTION OF THE REFRACTORY OXIDE PARTICLES IN THE POWDER MIXTURE.

United States Patent Oflice PREPARATION OF POWDER COMPOSITION FOR MAKING DISPERSION STRENGTHENED BINA- RY AND HIGHER NICKEL BASE ALLOYS Bud W. Kushnir, Edmonton, Alberta, Leon F. Norris and Robert W. Fraser, Fort Saskatchewan, Alberta, and David J. I. Evans, North Edmonton, Alberta, Canada, assignors to Sherritt Gordon Mines Limited, Toronto, Ontario, Canada No Drawing. Filed Aug. 21, 1969, Ser. No. 852,110

Int. Cl. B22f 9/00 US. Cl. 75-.5 AC Claims ABSTRACT OF THE DISCLOSURE Metal-refractory oxide powder compositions adapted for powder metallurgical fabrication of wrought dispersion strengthened nickel base binary and higher alloys are prepared by wet attritor grinding blends of very finely divided metal powders. The blends are formed of composite nickel-refractory oxide powder and chromium, tungsten, molybdenum and/or cobalt powder having a particle size below 1 micron. The wet attritor grinding is conducted for a time sufficient to produce a homogeneous powder mixture and to effect uniform spatial distribution of the refractory oxide particles in the powder mixture.

This invention is concerned with a method for preparing powder metallurgy compositions which are particularly adapted for fabrication of wrought dispersion strengthened nickel base binary and higher alloys by powder metallurgy techniques.

Dispersion strengthened lead, aluminum and nickel are well known and in commercial use in applications where the stability of their strengths at high homologous temperatures is necessary to meet specific materials engineering requirements. Generally, each of these materials is made from specially prepared metal powder compositions that have the dispersed phase, generally submicron metal oxide particles, incorporated into each metal powder particle such that it is mechanically inseparable therefrom. Up to the present time, however, these methods of powder preparation have not been successfully applied to the preparation of dispersoid containing metal alloy powders; thus effective dispersion strengthening has been restricted to single metal matrices.

The principal object of the present invention is to provide a method for the preparation of dispersoid containing metal powder compositions suitable for the fabrication if dispersion strengthened nickel base binary and higher alloy matrices in order to obtain the cumulative benefits of the dispersion strengthening and metal alloying mechanisms For example, the strength of nickel can be increased substantially in the intermediate temperature range by alloying it with molybdenum or tungsten; also, the oxidation resistance of nickel can be markedly improved by alloying it with chromium and each of these improvements is desired in dispersion strengthened nickel.

According to the present invention, this and other objects of the invention are achieved by providing a nickel powder having a mean particle size below 5 microns and preferably between 0.2 and 1 micron and containing from 0.5 to 40% by volume of submicron sized refractory metal oxide particles, blending said refractory oxide containing nickel powder with up to about 30% by weight of at least one finely divided alloying metal selected from the group consisting of chromium, tungsten, molybdenum and cobalt, said alloying metal being in metallic form or in a form reducible to metallic form and having a particle size below 1 micron and preferably between 0.1 to 0.5 micron, wet attritor grinding 3,734,713 Patented May 22, 1973 the so-formed powder blend for a time sufficient to produce a substantially homogeneous powder mixture and to effect a substantially uniform spatial distribution of the refractory oxide particles in the said powder mixture, recovering and drying said wet attritor ground blend, and heating the dried blend in contact with hydrogen to reduce oxygen in excess of that contained in the refractory oxide to below about 500 parts per million.

Powder compositions produced by this method may be readily fabricated into high strength homogeneous nickel base alloys containing a uniform dispersion of refractory oxide particles by powder metallurgy methods involving compacting, sintering and hot and cold mechanical Workmg.

In preparing the powder compositions according to the present invention, it is preferred to utilize composite nickel-refractory oxide powders in which the refractory oxide is uniformly distributed in the nickel powder and is mechanically inseparable from the individual nickel particles. Such powders may be prepared by direct hydrogen reduction of an aqueous slurry of refractory oxide impregnated basic nickel carbonate as described in detail in Canadian Pat. No. 786,268. According to the process, the aqueous basic nickel carbonate slurry preferably is prepared by distilling a nickel ammine carbonate solution and then it is thoriated by adding a desired amount of refractory oxide aquasol directly thereto. The oxide particles are adsorbed onto the surfaces of the basic nickel carbon-ate particles and after direct reduction of the nickel carbonate with hydrogen, the oxide particles are uniformly distributed over and fixed in the surfaces of the nickel powder particles. Preferred nickel-dispersoid powders prepared by this method consist of very fine nickel particles about 0.1-0.5 micron in size having about 0.5 to 4.0 weight percent of 10 to 50 millimicrons thoria particles embedded in this surfaces. The small thoria-containing nickel particles may occur singly or as loose agglomerates up to 5 microns in size.

Nickel-refractory oxide composite powders prepared by other methods may also be 'utilized in this invention. For example, nickel-thoria powder prepared by ball milling thoria sol with carbonyl or hydrogen reduced nickel powders having a mean particle size below about 2 microns have been found suitable.

The fine alloying metal powders employed in the method of the invention may be prepared by attritor grinding high purity commercial grade metal powders. Also, the alloying metals may be provided in the mixture in the form of extremely finely divided reducible compounds such as anhydrous tungstic acid and molybdic anhydride. However, where such compounds are utilized, it is essential to ensure that they are substantially completely reduced to elemental metallic form by heating in dry hydrogen preparatory to and/ or during fabrication of the powder composition into wrought form. Whether in elemental or compound form, it is essential that the alloying metal powder have a particle size below about 1 micron and preferably in the range of about 0.1 to 0.5 micron. With coarser particles, uniform spatial distribution of the refractory oxide particles cannot be achieved and, also, extremely long sintering times will be required in order to obtain homogeneity in the wrought products fabricated from powder blends containing coarse alloying metal particles.

The wet attritor grinding/mixing time is a critical variable in the method of the invention. The actual optimum grinding time will vary for different powder compositions. In general, grinding must be continued for a sufficient period of time to ensure that the different metallic powder constituents are substantially completely blended and, most importantly, until a substantially uniform spatial distribution of the refractory oxide constituent in the powder blend is obtained. That is, the grinding is continued to break down any refractory oxide particle agglomerates and to effect a re-distribution of the oxide particles in the blend so that the oxide inter-particle spacing is determined primarily only by the size of the alloying metal particles in the powder blend. In most cases, from about 15 to about 50 hours of attritor milling will be required to obtain optimum results.

On completion of the wet attritor grinding operation, the powder product is dried and then heated in hydrogen at an elevated temperature, preferably in the range of 1500 F. to 2000 F. to reduce the oxygen content of the blend such that the oxygen in excess of that contained in the refractory oxide constituent of the blend is less than 500 parts per million and preferably less than 200 parts per million. In most cases, this reduction operation will be effected during the sintering of the powder composition preparatory to fabrication by hot working such as by hot rolling or extrusion, for example.

The invention is illustrated by the following examples:

EXAMPLE 1 A 300 gram charge of nickel-thoria powder and fine chromium powder was mixed with water and attritor ground. The nickel-thoria powder, which was prepared in accordance with the method of Canadian Pat. No. 86,- 268, contained 2.7 wt. percent thoria and had a Fisher No. of 1.5 and an apparent density of 1.2 gm./cc. The fine chromium powder was prepared by attritor grinding commercially available chromium powder to produce a powder having a Fisher No. of 1.0. Samples of the blend (100 grams each) which had a nOminal composition of Ni/19Cr/2.1Th0 were removed after 2, 4, 8, 24, 48, 72 and 96 hours of attritor grinding and dried. The resulting powders were heated in hydrogen at 200 F./hr. to 2010 F. compaced into 30 gram compacts, sintered 30 hours at 2055 F. in hydrogen and fabricated into wrought strips by a hot rolling operation.

The ultimate tensile strengths at 2000 F. for the seven alloys are shown in the following table:

UTS at 2000 Grinding/mixing time (hours): F. Ks.i.

Average of 3 specimens.

The strength improvement with increasing grinding/ mixing time is clearly shown. The strength increased from about 12,00 p.s.i. with 2 hours grinding/mixing to a plateau at over 18,000 p.s.i. after 24 hours. Grinding/ mixing for over 24 to 48 hours appeared to offer no further improvement. The reason for this marked property improvement was studied using quantitative electron microscopy. The results indicated that the thoria particle density in the wrought alloy increased with milling time. Since particle density is a measurement of particle size and spatial distribution, it has been concluded that the strength improvement associated with increased attritor wet grinding/mixing is due to the decreased thoria particle size and a decreased interparticle spacing.

EXAMPLE 2 A master blend of Ni/Cr/W/Th0 powder having a nominal composition of Ni/18Cr/10W/2.5Th0 was prepared as follows: Charges of Ni/3.5Th0 and anhydrous tungstic acid were attritor ground/mixed in water for 24 hours. The products were dried and then reduced in 4 flowing H for 1 hour at 1500 F. The Ni/W/ThO was then wet attritor ground/mixed with 1.0 micron chromium powder for an additional 24 hours. The dried products were heated in H for 30 hours at 2055 F. to reduce the oxygen in excess of that in the thoria to'400 p.p.m.

Samples of powder were formed into 30 gram compacts and fabricated into strip by hot rolling. Average ultimate tensile strength of these alloys at 2000 F. was

18,000 p.s.i.

EXAMPLE 3 A Ni/Mo/Th0 powder blend was prepared as follows: Nickel-2.7 thoria powder as used in Example 1 and molybdic acid were mixed to provide a molybdenum content of about 18 wt. percent. The powder mixture was slurried with water and attritor ground for 24 hours. The ground material was dired and then de-oxidized by heating in hydrogen at 1500 F. for 2 hours. The oxygen content in excess of that in the thoria was 200 ppm.

A sample of this powder was formed into a 30 gram compact, sintered and fabricated into strip by hot rolling at 2200 F. and annealing for 16 hours at 2450" F. The sample had a room temperature ultimate tensile strength of 160,000 p.s.i. and a 2000 F. UTS of 11,500 p.s.i.

EXAMPLE 4 A Ni/Cr/Mo/Th0 powder composition was prepared as follows: Weighed amounts of Ni/3.5ThO having a Fisher No. of 0.95 and an apparent density of 1.5 and Molybdic anhydride having a Fisher No. of 1.4 and an apparent density of 0.5 were attritor ground/mixed in water for 48 hours. The slurry was dried, reduced in flowing hydrogen for 3 hours at 18.00 F., then wet attritor ground/mixed with 1 micron chromium powder for an additional 48 hours and dried.

Samples of this powder which had a nominal composition of 18% Cr, 11% M0, 2.5% ThO balance Ni, were heated in hydrogen for 2 hours at 2000 F., compacted into 60 gram billets and sintered in hydrogen at 2050 F. for 60 hours. Excess oxygen in the sintered compacts was less than 200 ppm. Wrought material prepared from the sintered compacts by a cold roll-hot roll fabrication sequence has a homogenous microstructure and uniform fine thoria distribution. Average room temperature UTS of these materials was 120,000 p.s.i., 1200 F. UTS 100,- 000 p.s.i. and 2000 F. UTS 12,000 p.s.i.

We claim:

1. The method for preparing powder metallurgy compositions suitable for fabrication of wrought dispersion strengthened nickel base 'binary and higher alloys by powder metallurgy techniques which comprises providing a nickel powder consisting essentially of particles of a size below about 5 microns and containing from about 0.5 to 40% by volume of submicron sized, mechanically inseparable refractory metal oxide particles, mixing said refractory metal oxide containing nickel powder with up to about 30% by weight of at least one finely divided alloying agent selected from the group consisting of chromium, tungsten, moylbdenum, and cobalt, said alloying agent being inmetallic form or in the form of compounds of said metals which are reducible to metallic form by reaction with hydrogen at elevated temperatures below the melting point of said metals and having a particle size below about 1 micron, forming a slurry of the soformed powder blend in water, attritor milling said slurry for a time sufficient to produce a substantially homogenous powder mixture and to effect a substantially uniform spatial distribution of the refractory oxide particles within the powder mixture as indicated by a levelling off of the high temperature tensile strength of the wrought dispersion strengthened alloy fabricated from said milled powder mixture, recovering and drying said wet attritor ground powder mixture and heating the dried mixture in contact with hydrogen to reduce oxygen in excess of that contained in the refractory oxide to below about 500 parts per million.

2. The method according to claim 1 in which the refractory oxide containing nickel particles are in the size range between 0.2 and 1 micron.

3. The method according to claim 2 in which the alloying agent is chromium.

4. The method according to claim 2 in which the alloying agents are chromium and tungsten.

5. The method according to claim 1 in which the refractory metal oxide containing nickel particles contain from 0.5 to 4.0 weight percent of 10 to 50 millimicron thoria particles embedded in the surfaces thereof.

References Cited UNITED STATES PATENTS 3,070,440 12/1962 Grant et al 75206 3,249,407 5/1966 Alexander et a1. 75206 3,479,180 11/1969 Lambert et al 75206 0 WAYLAND W. STALLARD, Primary Examiner 

